Copying DNA
DNA replication is semiconservative: each new double helix keeps one old strand and gains one new strand. In bacteria it starts at one origin of replication. Helicase unwinds the helix, primase lays down short RNA primers, and DNA polymerase adds nucleotides only in the 5' to 3' direction. The leading strand is made continuously. The lagging strand is made in pieces called Okazaki fragments, which ligase joins.
Gene expression
Transcription makes an RNA copy of a gene. RNA polymerase binds a promoter, reads the template strand 3' to 5', and builds mRNA 5' to 3' using the base pairs A-U, T-A, G-C and C-G (template base to RNA base). Translation reads the mRNA in three-base codons on a ribosome. Each tRNA carries an amino acid and has an anticodon that pairs with a codon. AUG is the usual start codon and codes for methionine. UAA, UAG and UGA are stop codons. Bacteria have no nucleus, so ribosomes can start translating an mRNA while it is still being transcribed.
Mutations
A point mutation changes one base. It may be silent (same amino acid), missense (different amino acid) or nonsense (a stop codon appears early, giving a shortened protein).
Horizontal gene transfer
Bacteria also pass genes sideways, between cells that are not parent and offspring:
- Transformation. A cell takes up free DNA from its surroundings.
- Transduction. A bacteriophage carries a fragment of bacterial DNA from one cell to another.
- Conjugation. Two cells join through a pilus, and a plasmid (often an F plasmid) is copied across.
Worked example
A template strand reads 3'-TAC CCA GGG ATT-5'. Find the peptide.
- Pair each base: T gives A, A gives U, C gives G. The first codon is AUG. Continuing, the mRNA is 5'-AUG GGU CCC UAA-3'.
- AUG is methionine, GGU is glycine, CCC is proline, and UAA is a stop codon.
- Translation ends at the stop, so the peptide is Met-Gly-Pro, three amino acids.
Now suppose the second codon's template changes so that GGU becomes UGA. That is a nonsense mutation: the chain would end after methionine. Compare this with a silent change, such as GGU becoming GGC, which still codes for glycine.